1. Field of the Invention
This invention relates to the polymerization of olefins and more particularly relates to catalyst compositions useful for polymerizing one or more monomers comprising ethylene to polymers having a controlled molecular weight distribution and a good balance of physical properties.
2. Description of the Prior Art
It is known that catalysts of the type variously described as coordination, Ziegler, Ziegler-type, or Ziegler-Natta catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also known that the properties of the polymers obtainable by the use of such catalysts, as well as the relative economies of the processes used to prepare the polymers, vary with several factors, including the choice of the particular monomers, catalyst components, polymerization adjuvants, and other polymerization conditions employed.
During the years since Ziegler catalysts were first publicly disclosed, there has been a considerable amount of research conducted on the use of such catalysts; and numerous publications have resulted from that research. These publications have added much to the knowledge of how to make various types of olefin polymers by various types of processes. However, as is apparent from the amount of research on Ziegler catalysis that is still being conducted throughout the world, as well as the number of patents that are still being issued to inventors working in the field of Ziegler catalysis, the means of attaining certain results when polymerizing olefins with Ziegler catalysts are still frequently unpredictable. The fact that this situation exists is sometimes due to the need to obtain a previously-unattainable combination of results; occasionally due to difficulties in obtaining the same results in a commercial-scale apparatus as in a laboratory-scale reactor; and often due to a polymerization parameter's having an effect, or side-effect, in a given type of polymerization process that is different from effects achieved by its use in prior art processes of a different type.
One aspect of Ziegler catalysis in which the need for further research has been found to exist has been in the provision of catalyst compositions suitable for use in a commercially-feasible process for preparing ethylene polymers having a good balance of physical properties and a molecular weight distribution that can be controlled so as to make the polymers formable by whichever forming technique is intended to be employed for producing articles from the polymers, e.g., injection molding or blow molding.
There are, of course, known processes for preparing injection molding resins, as well as known processes for preparing blow molding resins, by polymerizing ethylene with the aid of Ziegler catalysts. However, the known processes typically suffer one or more of the disadvantages of lack of economy, inability to produce polymers having a suitable balance of properties, and/or unreliability in producing such polymers--particularly in commercial-scale operations. Moreover, the different molecular weight distribution requirements of polymers to be formed by different techniques have compounded the problem of finding a family of catalyst compositions suitable for use in a commercially-feasible process wherein the molecular weight distribution can be controlled so as to produce the desired polymers. What is still needed is a catalyst which (a) is suitable for use in a gas-phase polymerization process, (b) is capable of yielding polymers having a good balance of physical properties and a controlled molecular weight distribution, and (c) has sufficient activity to be economically attractive.
British Pat. No. 1,489,410 (Monsanto) teaches gas-phase polymerization processes which, because of their use of supported Ziegler catalysts having a vanadium component and other factors, are commercially attractive processes. However, as taught in the patent, the processes are designed only for the preparation of polymers having the relatively broad molecular weight distributions suitable for blow molding resins, i.e., molecular weight distributions such that their normalized V.sub.30 /V.sub.300 melt viscosity ratios are above 2.3. Moreover, it has been found that these processes, although useful for preparing blow-molding resins of the type employed for household chemical containers, do not appear to be adaptable to the preparation of blow-molding resins requiring somewhat narrower molecular weight distributions, e.g., liquid food containers, or to the preparation of injection-molding resins requiring still narrower molecular weight distributions.
Attempts to make the processes of the Monsanto patent suitable for the preparation of injection molding resins, as well as more reliable in the preparation of desirable blow molding resins, by combining the teachings of the patent with the teachings of publications that discuss means of narrowing molecular weight distribution have not been successful. For example, polymers having a sufficiently narrow molecular weight distribution have not been obtained when Monsanto's preferred vanadium halides have been replaced with the alkoxy group-containing vanadium compounds which are within the scope of their patent and which U.S. Pat. Nos. 3,457,244 (Fukuda et al.) and 3,655,583 (Yamamoto et al.) teach to result in the production of polymers having narrower molecular weight distributions when unsupported catalyst systems are employed.
British Pat. No. 1,175,593 (Stamicarbon) teaches a process for preparing ethylene/higher alkene/polyunsaturated compound terpolymers by the use of an unsupported vanadium chloride/alkylaluminum halide catalyst system, the activity of which is increased by adding an alcohol or phenol to the vanadium compound and/or the aluminum compound. According to the speculative teachings of the patent, the alcohol may be a polyhydric alcohol in which the --OH groups are not attached to adjacent carbon atoms, but the patent does not mention any particular polyhydric alcohol that might be used or suggest the effect that the inclusion of a polyhydric alcohol might have on molecular weight distribution if the catalyst composition were being used for the preparation of crystallizable ethylene polymers, such as injection molding or blow molding resins, rather than the rubbers of the patent.